I can't understand that when AgNO3 Is added to KI solution ,ppt. adsorbes I- ions leading to the formation of negatively charged colloidal solution ,but when KI is added to same AgNO3 solution ,then adsorption of Ag+ ion takes place ,why not I- get adsorbed in this case also?Kindly explain

The charge on the colloidal particles is due to the preferential adsorption of either positive or negative ions on their surface.

 If the positive ions get adsorbed then the sol particles acquire a positive charge and if negative then the sol gets negative charge.

It is important to note that the ion which is more closely related chemically to the colloidal particle is preferentially adsorbed by it. Sometimes it is possible to have positively as well as negatively charged sol of the same substance.

 

(i) When a dilute solution of silver nitrate is added to a slight excess of dilute solution of potassium iodide, a negatively charged sol of silver iodide is formed (I- ion is preferred here). It is due to the adsorption of iodide ions (I-) from the dispersion medium on the precipitate of silver iodide (AgI). AgI + Iāˆ’ ā†’ [AgI]I-  (negatively charged AgI sol)

(ii) When a dilute solution of potassium iodide is added to a slight excess of a dilute solution of silver nitrate, a positive charged sol of silver iodide is formed (Ag+ ion is preferred here). It is due to the adsorption of Ag+ ions from the dispersion medium on the precipitate of silver iodide (AgI) .

AgI + Ag+ā†’ [AgI]Ag+  (positively charged AgI sol)

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